Optical imaging is one of the cornerstones of modern technology. One of the outstanding problems in the field is the limited depth-of-field (DOF) of imaging systems and limited range of focusing distance of illumination systems. DOF will be used to indicate both DOF for imaging applications and range of focusing distance for illumination applications.
In general solutions that increase DOF can be cumbersome; they involve relative motion of component parts (such as lenses) or substitution of parts into and out of an apparatus such as a camera or a light source fixture for illumination. Many methods have been proposed to increase DOF of imaging systems. These include: increasing the f-number of optics by reducing it's aperture, ‘annular aperture’ methods, in which focal depth is extended by obstructing the center part of light; ‘shade mask’ methods, in which focal depth is extended by modulation of the amplitude transmittance over the whole pupil aperture; ‘quasi-bifocus’ methods, in which focal depth is extended by the generation of bifocus; and image-processing methods. Apparatus using these methods suffer from disadvantages such as loss of incident light energy and reduction of imaging resolution.
Accordingly, an apparatus that posses inherently high DOF remains highly desirable.